<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>

<div id="a">
    hello huahua
</div>

<button id="btn">click me</button>

<script>
    document.getElementById("btn").onclick = function(){
            document.getElementById("a").innerHTML = "haha"
    }
</script>

</body>
</html>